Referrer API must return correct Content-Type

As per the spec, Content-Type header MUST be set to application/vnd.oci.image.index.v1+json

This is important as GAR today (OCI 1.0) redirect "referrers" API call to the authentication page, and doing so the call return 200 OK  + content-type: text/html.

pkg/v1/remote/referrers.go

@@ -66,7 +66,7 @@ func (f *fetcher) fetchReferrers(ctx context.Context, filter map[string]string,
 	}
 
 	var b []byte
-	if resp.StatusCode == http.StatusOK {
+	if resp.StatusCode == http.StatusOK && resp.Header.Get("Content-Type") == string(types.OCIImageIndex) {
 		b, err = io.ReadAll(resp.Body)
 		if err != nil {
 			return nil, err